The permanent magnet of R--T--B system chiefly comprising Fe has a high performance, so that demand for this magnet has been increasing year after year in the market. However, this magnet has an inherent problem that its corrosion resistance is inferior to that of a permanent magnet of Sm-Co system or a ferrite magnet.
The conventional countermeasure against the above problem is to provide a protective layer by a resin coating by spray or electro-deposition, or vapor plating such as vapor deposition or ion sputtering. Above all, Ni plating is widely used due to its superb corrosion resistance. (Japanese Patent Application Laid-open No.54406).
Nevertheless, the Ni plating film is hard to grow in the grain boundary (Nd-rich phase), because Nd is highly susceptible to oxidation and is rapidly dissolved into the plating bath.
Furthermore, a cell structure appears on the surface of the Ni plating film because fine irregularities exist in the surface of the permanent magnet of R--T--B system, and the Ni plating film does not grow uniformly. The double points and triple points in the boundaries of the cell structure in the surface of this Ni plating film often become start points of corrosion.
The cell boundaries of]the surface of the Ni plating film are formed like a valley or indentations, so that dirt is liable to adhere thereto after the film is formed. The dirt is one of the factors causing corrosion to occur.